BOSTON - Two explosions ripped through the Boston Marathon's crowded finish line Monday afternoon, killing at least three people and injuring more than 141.

The dead included an 8-year-old boy, The Boston Globe reported, citing law enforcement sources briefed on the investigation. Among the injured, 17 were reported in critical condition. The victims at eight local hospitals were as young as 2.

"There were so many people in that area that they couldn't get ambulances in there," said Joe Difazio, who was working on communications near the site when the blasts occurred. "They were wheeling people out in wheelchairs. One guy had no legs. The bones was just sticking out. ... It was horrible."

Set off at one of the world's premiere sporting events, the blasts ignited a fresh round of unease and renewed security concerns across the nation. The stark pictures of mayhem and the injured sent over TV and the Internet also rekindled stark memories from the 9/11 terrorist attacks on New York's World Trade Center and the Pentagon. Monday's attacks also come just four days before the anniversary of the April 19, 1995, bomb attack on Oklahoma City's Murrah Federal Building.

The Boston Marathon blasts occurred about 12 seconds and 100 yards apart at about 2:50 p.m., three hours after Ethiopia's Lelisa Desisa won the 26.2-mile race.

Police searched widely for other explosive devices, and two law enforcement officials said no other explosive devices had been found. The officials said that as many as seven suspicious packages were destroyed in controlled explosions but they were later found not to be actual bombs. The law enforcement officials who have been briefed on the matter were not authorized to comment publicly.

Authorities also issued a bulletin for a unidentified man who was seen running from a restricted area. It was unclear, according to the federal official, whether the person was anything other than a witness.

The state, local and federal investigation into the bombings is being led by the FBI. Special Agent Rick Deslauriers said investigators were looking at possible terrorism, but he refused comment on specific suspects or leads in the case.

"It is a very active, fluid investigation at this time,'' he said.

Speaking from the White House, a somber President Obama said people should not speculate over who was responsible.

"We still don't know who did this or why. People should not jump to conclusions before we have all the facts. But make no mistake. We will get to the bottom of this. We will find out who did this. We will find out why they did this. Any individual or responsible groups will feel the full weight of justice."

The devices were described by the official as relatively small and possibly containing small ball bearings or BB gun pellets designed to serve as shrapnel. It was unclear whether the devices were remotely detonated or included timers, the official said, adding that no conclusions had been drawn on whether an organized group or lone wolf had been responsible for the attack.

Multiple media reports indicated a person of interest was being held at an undisclosed hospital. But authorities later dismissed those reports. Davis said investigators are talking to several people but "there is no suspect at this time."

A third incident at the John F. Kennedy Presidential Library was initially described as a third explosion, but Davis said Monday night that it may have been only a fire. No injuries were reported, but nearby universities were being evacuated.

"After this incident occurred, there were a lot of people running from the scene, a lot of them deposited bags and parcels," Davis said. "Each one is being treated as a suspicious device. At this point, we haven't found any more devices."

Bloodied spectators were carried to a medical tent intended for runners. At least one police officer was hurt.

Organizers stopped the race and locked down the marathon headquarters.

The Federal Aviation Administration announced a temporary flight restriction over Boston.

Cellphone service was operating, wireless companies reported, contrary to an earlier Associated Press report quoting a law enforcement official who said service was cut in case there were other undetected devices.

The elite women runners started the race at 9:30 a.m., and the elite men followed about 30 minutes later. About 27,000 runners were in the field for the Patriots' Day race.

Nancy Costa, a medical student from Reading, Pa., was running with her friend Jill Edmonds of Salem, N.H., when the explosions erupted.

"It was insane here. Everyone was running. I was right next to the explosion. It threw me," she said. "I never sprinted so fast after a marathon.

"The first (blast) threw me onto the ground. And everything went silent and then the second went off and I just covered my head and got up and started sprinting. Everyone was screaming and people were getting trampled. We finally found an open T (subway train) that just arrived in Wellington (station). We had to walk a few miles to find one open."

Kimberly DelGuzzi of Pittsburgh was waiting on Boylston Street for her friend to cross the finish line when she found herself pressed against a building, ducking for cover from the blasts.

"At first, I thought it was fireworks, but then I saw the smoke go up in the air," said DelGuzzi, who was standing between the two explosions. "Then, not even a minute later, the second one went off."

She described the scene as "mass chaos" and said, "Oh my God, it was loud."

"The explosions shook everything," she said, her voice still shaking 40 minutes after the bombs went off. "I saw runners down in the street. I saw people down on the sidewalk."

DelGuzzi, 41, has run numerous marathons but was not running in Boston. Her friend reported to her that she was OK.

Jim Davis, one of the marathon's official photographers, told TheDes Moines Registerhe was about 50 feet from where the first blast ripped through a glass storefront.

"Debris was falling. Fortunately I was far enough away that there weren't any glass shards," said Davis, 65, of Fairfield, Iowa. "Then people started running and screaming and I realized this is not an accident - I should get out of there."

After the second explosion, about a block away, Davis returned for his camera gear and saw one man who had lost both his legs and others who were severely cut.

"I'm not a war correspondent," he said. "I'm not used to seeing people blown up with injuries."

Tom Beusse, president of the USA TODAY Sports Media Group, had just finished the race and was about 150 yards away from the explosion.

"There was this giant explosion. All of us turned around, the runners, and had these looks on their faces like 'Oh my God.' ... Immediately, it turned into mayhem. People were screaming. Cops told us to keep moving away from the finish line in the direction we were going. No one knew what was coming next - and thankfully, nothing was next."

Massachusetts General Hospital was treating 19 victims, spokeswoman Susan McGreevey said. Six were in surgery in critical condition, four suffering "traumatic amputations" from having legs cut off by the force of the explosions.

As night fell at Brigham and Women's Hospital, a Level 1 trauma center, three police officers with rifles stood guard at the front entrance. Inside, physicians cared for 28 victims of the Boylston Street blast, including two in critical condition, two at risk of losing limbs and nine who needed surgery.

Staff throughout the hospital felt the impact of the day's events. Social worker Laura Taylor was among those not allowed to leave at 4 p.m. due to a lockdown to ensure security, but instead stayed past 7 p.m. As she checked on patients, other social workers and psychologists tended to family members in expanded waiting areas.

"We've had other emergencies over the seven years I've worked here, but nothing where we had to go into lockdown," Taylor said. "This had a level of intensity greater than anything else, just knowing what was happening outside."

Others gathered outside to comfort one another. Marathoner Kirsten Scott was still wearing her number as she talked with a friend and thought about her husband, a surgery resident tending to patients. "We're just praying," she said.

The most common injuries were to bones and tissues. Among the youngest, a 3-year-old was transferred to Boston Children's Hospital for treatment.

Michael Malcolm, a concierge at the Bryant Back Bay apartment building who was a couple of blocks from the race finish line, heard the blasts, but said they didn't register until he saw crowds of runners coming up the street, some knocking on the front door of the building asking if they could use his telephone or plug in their cellphones to let loved ones know they were safe.

At one point, a perpetual wail of sirens was so loud that people had to shout when talking to one another, he said.

"It's scary because we're not used to things happening like this in this city," said Malcolm, 21.

Police evacuated a 15-block area around Boston's Copley Square and bolstered security around the city.

Security was also increased around Washington and New York.

The New York Police Department has stepped up security around landmarks in Manhattan, including near prominent hotels, said Paul Browne, deputy commissioner of the NYPD.

"New Yorkers should be alert. And we ask them to be aware of their surroundings and if they see anything suspicious, call our resources, call 911," New York City Police Commissioner Raymond Kelly said.

"We've had the experience here of significant events and plots against us, so we would hope that New Yorkers are vigilant," said Kelly, referring to the 9/11 terrorist attack, the 1993 World Trade Center bombing and other major incidents. "We are in the cross hairs of terrorists, and we don't believe the threat has diminished very much. We don't know the genesis of this. We don't know who's responsible for it. But we sort of have to have a 360-degree perimeter in this city and other cities as well, obviously."